High-strength aluminum casting alloys resistant to hot tearing

ABSTRACT

An aluminum casting alloy resistant to hot tearing includes, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about 3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc, about 0.18 to about 0.28 Zr, and the balance Al and impurities, substantially excluding Fe, Mn, and Si, said alloy characterized by a freezing range of less than about 150° C., solidus temperature above about 490° C., and eutectic phase fraction above about 5% at the late stages of solidification. The alloy is processed to form a dispersion of L1 2  particles inoculating fcc grains with a grain diameter of about 40 to about 60 μm, and η′-phase precipitates enabling an ambient yield strength from about 410 MPa to about 540 MPa.

GOVERNMENT INTERESTS

Activities relating to the development of the subject matter of this invention were funded at least in part by United States Government and thus may be subject to license rights and other rights in the United States, specifically contract number FA8650-05-C-5800.

BACKGROUND OF THE INVENTION

The 7XXX wrought Al—Zn-based alloys are commonly used in structural applications demanding high specific strength. Compared to wrought alloys, castings decrease the fabrication cost and associated logistics lead time, because castings enable near-net-shape products. However, the known 7XXX alloys are susceptible to hot tearing during solidification and therefore not optimal for casting. The hot tearing is caused by a relatively high thermal expansion coefficient and significant volumetric difference between liquid and solid.

Senkov et al. [U.S. Pat. No. 7,060,139 (incorporated by reference herein)] disclose a high-strength aluminum alloy with a nominal composition of Al—6.0˜12.0 Zn—2.0˜3.5 Mg—0.1˜0.5 Sc—0.05˜0.20 Zr—0.5˜3.0 Cu—0.10˜0.45 Mg—0.08˜0.35 Fe—0.07˜0.20 Si, in wt %. The alloy by Senkov et al. shows high tensile strength while maintaining high elongation in ambient temperatures and cryogenic temperatures. The freezing range of the alloy by Senkov et al. is about 164 to about 195° C., the solidus temperature about 422 to about 466° C., and the eutectic phase fraction about 1.1 to about 1.5%. However, the alloy shows poor casting characteristics. Thus, there has developed a need for new 7XXX aluminum casting alloys that are resistant to hot tearing. Such alloys would be useful for articles of manufacture such as hydrogen turbo pump housing or other aerospace materials.

SUMMARY OF THE INVENTION

In a principal aspect, the present invention comprises high-strength aluminum casting alloys that are resistant to hot tearing. The yield strength of the casting alloys ranges from about 410 MPa to about 540 MPa, at room temperature. The invented alloys are Al—Zn-based and comprise the major alloying elements Sc, Zr, Mg, and Cu. The amounts of Sc and Zr are optimized to produce primary L1₂-phase particles which refine the grain size and improve the hot-tearing resistance as well as fatigue resistance and toughness. The amounts of Zn, Mg, and Cu are optimized for high resistance to hot-tearing and high strength. The amounts of Fe, Mn, and Si are kept low and at a minimum because these elements have a detrimental effect on strength and hot-tearing resistance.

To produce primary L1₂-phase particles, the solvus temperature of the L1₂ phase must be above the solvus temperature of the fcc phase. The solvus temperatures can be computed with thermodynamic database and calculation packages such as Thermo-Calc® software version N offered by Thermo-Calc Software. Alternatively, in the composition space of the alloys, the solvus temperatures can be approximated by the following equations:

L1₂ solvus=87.01×wp_(Sc)+157.89×wp_(Zr)−243.43×wp_(Sc)×wp_(Zr)+267.06×wp_(Sc) ^(0.14)+769.51×wp_(Zr) ^(0.05)

fcc solvus=−1.76×wp_(Zn)−5.14×wp_(Mg)−0.005×wp_(Zn)×wp_(Mg)+139.13×wp_(Zn) ^(0.002)+792.11×wp_(Mg) ^(0.0002)

where wp_(Sc), wp_(Zn), wp_(Zr), and wp_(Mg) are the weight percentages of Sc, Zr, Zn, and Mg, respectively. These equations are based on the best fit for solvus temperatures.

Additionally, the amount of Zr is kept below about 0.3 wt % to minimize the formation of Al₃Zr which has a D0₂₃ crystal structure. As shown by Hyde in Al—0.5Sc—0.4Zr (wt %), D0₂₃ particles quickly grow too large [Hyde, K. 2001. The Addition of Scandium to Aerospace Casting Alloys. Ph.D. diss., University of Manchester (incorporated herewith)], and are not very effective for refining the fcc grain size. In the discovered alloys, small Al₃(Sc, Zr) particles with an L1₂ crystal structure are employed instead to inoculate small fcc grains during melt cooling. Because Zr is an inexpensive substitute for Sc in L1₂, the alloys of the invention use as much Zr as possible, about 0.25±0.05 wt %. However, where cost is not a limiting factor, as little as 0.15 wt % Zr can be used in combination with a larger amount of Sc.

The amounts of Sc and Zr in the casting alloys are optimized for cooling rates up to about 100° C. per second. The L1₂-Al₃(Sc, Zr) particle size distribution depends on the melt cooling rate. Casting into a sand mold results in a cooling rate of about 0.5° C. per second. Higher cooling rates are accessible through direct-chill casting where the billet is cooled, for example, with water during solidification. Cooling rates above about 100° C. per second are accessible through casting methods such as the Continuous Rheoconversion Process (CRP).

As shown in FIG. 1, large primary L1₂ particles will result in fcc grains larger than 200 μm in diameter. To achieve an fcc grain diameter of about 40 to about 60 μm at cooling rates up to about 100° C. per second, the mean radius of primary L1₂ particles should be less than about 2 μm and its phase fraction should be less than 1% by weight.

FIG. 1A shows the amounts of Sc and Zr which enable the required L1₂ particle size. Because the amount of Zr is kept below about 0.3 wt %, the amount of Sc is kept above about 0.4 wt %, up to about 0.6 wt %.

Because hot tearing is caused substantially by a thermal contraction during solidification, resistance to hot tearing can be improved by decreasing the freezing range and increasing the solidus temperature below which the aluminum alloy is completely solid. It is also helpful to increase the eutectic phase fraction formed at late stages of solidification, because the eutectic phase solidifies completely at one temperature and reduces the amount of melt contracting over the freezing range.

Solidification parameters such as the freezing range, the solidus temperature, and the eutectic phase fraction can be computed with thermodynamic database and calculation packages such as Thermo-Calc software. To compute solidification parameters of complex alloy systems with Thermo-Calc software, the Gibbs free energy of relevant phases must be assessed following the CALPHAD (CALculation of PHAse Diagrams) approach. One such relevant phase is the metastable η′ phase, because the 7 XXX wrought alloys employ q′ phase precipitates for strengthening. For efficient strengthening, the mean radius of η′ precipitate should be less than about 5 nm.

The η′ phase precipitation kinetics can be simulated with PrecipiCalc® software version 0.9.2 offered by QuesTek Innovations LLC after assessing the thermodynamic description. The predicted particle size distribution can be used as input to a mechanistic model of the yield strength, which comprises contributions from precipitation strengthening, grain-size strengthening, solid-solution strengthening, and dislocation strengthening. The amounts of Zn, Mg, and Cu of the alloys are chosen to optimize the solidification parameters at various yield strength levels.

The amounts of Fe, Mn, and Si are kept as low as possible because these elements otherwise form large insoluble constituent particles of Al₁₃Fe₄, Al₇Cu₂Fe, Mg₂Si, and Al₆Mn which negatively affect the toughness, fatigue, and SCC resistance. The amount of Fe is preferably kept below about 0.0075 wt %, Mn below about 0.2 wt %, and Si below about 0.03 wt %.

In order to avoid incipient melting during homogenization or solution treatment, the homogenization or solution treatment temperature should be below the final solidification temperature, preferably with a safety margin of about 10 to 30° C. A two-step treatment distinguishing the homogenization from the solution treatment, as shown in FIG. 3, can introduce an additional safety factor to avoid incipient melting. The calculated final solidification temperature is about 493° C. Thus, in one embodiment, the homogenization and solution treatment should be at about 460 to 480° C. The time of such treatments should be long enough to eliminate the majority of as-cast segregation. As shown in FIG. 4, homogenization simulations show that a homogenization at 460° C. for 2 hours followed by a solution treatment at 480° C. for 1 hour should be sufficient to eliminate the majority of as-cast elemental segregation. This simulation was conducted with the kinetic software DICTRA™ (DIffusion Controlled TRAnsformations) version 24 offered by Thermo-Calc Software.

The subject matter of the invention is applicable to aluminum 7XXX alloys in particular, but the invention is not necessarily so limited. Thus, one benefit of the invention is to eliminate, or substantially eliminate, hot tearing of cast aluminum alloys.

Further benefits, advantages and features of the invention are set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description which follows references will be made to the drawing comprising the following figures:

FIGS. 1A and 1B respectively are graphs depicting the simulated primary L1₂ particle radius and simulated grain size as a function of the alloy Sc and Zr;

FIGS. 2A, 2B, and 2C respectively are graphs depicting strength and solidification parameter contours as a function of Zn, Mg, and Cu content wherein the following legends are utilized:

______ Yield strength iso-contours (ksi)

% Eutectic (Scheil)

______ Scheil Freezing range (° C.)

- - - Scheil solidification temperature (° C.)

Star: High strength solution (YS˜80 ksi)

Triangle: Medium strength solution (YS˜70 ksi)

Square: Low strength solution (YS˜60 ksi)

FIG. 3 is a time-temperature diagram illustrating the processing steps for processing an embodiment of the alloy of the invention; and

FIG. 4 is a homogenization simulation of the examples of the invention.

FIG. 5 is a micrograph of alloy A of the invention. The micrograph is typical of the examples of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Following are specific examples of the invention.

Example 1 Alloy A

A melt was prepared comprising Al—6.3 Zn—3.2 Mg—1.1 Cu—0.52 Sc—0.20 Zr, in wt %. The exemplary alloy preferably includes a variance in the constituents in the range of plus or minus ten percent of the mean value. The alloy was cast through the CRP reactor into a sand-casting mold at measured cooling rates of 50˜100° C./second. As shown in FIG. 3, the optimum processing condition was to apply hot isostatic pressing, homogenize and solutionize at 460° C. for 2 hours and 480° C. for 1 hour, quench with water, hold at room temperature for 24 hours, and age at 120±10° C. for 20 hours. The ambient yield strength in this condition was 521±12 MPa. The grain diameter was about 50 μm, or an ASTM (American Society for Testing and Materials) grain size number of about 5.7. The calculated freezing range is 136° C., solidus temperature 493° C., and the eutectic phase fraction formed at late stages of solidification is 10%.

A rectangular panel of alloy A was cast successfully without hot tearing. The melt was degassed with argon for 45 minutes at 700˜720° C. and then reheated to 740° C. just prior to mold pouring. The mold measured about 1 cm in depth. The pouring time to fill the mold was approximately 20 seconds. The mold filled successfully, producing a panel suitable for characterization. Following the breakout from the mold, removal of all gating and cleaning, the panel was shipped to UES, Inc. at the Wright Patterson Air Force Base for characterization. FIG. 5 shows the microstructure of alloy A, where pores from casting, an exemplary L1₂ particle, and the eutectic phase are marked as a, b, and c, respectively.

Example 2 Alloy B

A melt was prepared comprising Al—5.3 Zn—3.0 Mg—1.1 Cu—0.55 Sc—0.25 Zr, in wt %. The exemplary alloy preferably includes a variance in the constituents in the range of plus or minus ten percent of the mean value. The alloy was cast through the CRP reactor into a sand-casting mold at a measured cooling rate of 100° C./second. As shown in FIG. 3, the optimum processing condition was to apply hot isostatic pressing, homogenize and solutionize at 460° C. for 2 hours and 480° C. for 1 hour, quench with water, hold at room temperature for 24 hours, and age at 120±10° C. for 20 hours. The ambient yield strength in this condition was 482±6 MPa. The grain diameter was about 54 μm, or an ASTM grain size number of about 5.5. The calculated freezing range is 139° C., solidus temperature 494° C., and the eutectic phase fraction formed at late stages of solidification is 9%. A rectangular panel of alloy B was cast successfully without hot tearing in accord and otherwise generally with the protocol of alloy A.

Example 3 Alloy C

A melt was prepared comprising Al—4.5 Zn—2.3 Mg—0.62 Cu—0.42 Sc—0.25 Zr, in wt %. The exemplary alloy preferably includes a variance in the constituents in the range of plus or minus ten percent of the mean value. The alloy was cast through the CRP reactor into a sand-casting mold. As shown in FIG. 3, the optimum processing condition was to apply hot isostatic pressing, homogenize and solutionize at 460° C. for 2 hours and 480° C. for 1 hour, quench with water, hold at room temperature for 24 hours, and age at 120±10° C. for 15 hours. The calculated ambient yield strength in this condition is 410±40 MPa. The calculated grain diameter is about 50 μm or an ASTM grain size number of about 5.7. The calculated freezing range is 145° C., solidus temperature 494° C., and the eutectic phase fraction formed at late stages of solidification is 6%. Two panels were successfully cast from one heat of alloy C without hot tearing and otherwise generally in accord with the protocol used for alloy A.

Table 1 summarizes the compositions of the examples set forth above and sets forth the general range of the constituents for the practice of the invention in weight percent:

TABLE 1 Range Alloy A Alloy B Alloy C Zn 4.0-6.9 5.8-6.8 4.8-5.8 4.0-5.0 Mg 2.0-3.5 2.9-3.5 2.7-3.3 2.0-2.6 Cu 0.6-1.2 1.0-1.2 1.0-1.2 0.52-0.72 Sc 0.38-0.57 0.52 0.55 0.42 Zr 0.18-0.28 0.20 0.25 0.25 Al Balance Balance Balance Balance Fe <0.0075 <0.0075 <0.0075 <0.0075 Mn <0.2    <0.2    <0.02   <0.2    Si <0.03   <0.03   <0.03   <0.03  

Table 2 summarizes the information with respect to the microstructural elements of the examples set forth above and considered relevant to the range of the constituents in the practice of the invention.

TABLE 2 Solidus Temp About 490° C. or higher Freezing Range About 150° C. or lower Eutectic Phase Fraction About 5-15% Phases fcc, L1₂ < 1% by weight, and η′ fcc Grain Size About 40-60 μm Mean Particle Size (η′) Less than about 5 nm Yield Strength About 410-540 MPa

While embodiments of the invention have been disclosed, the scope thereof is not so limited and the invention is to be limited only by the following claims and equivalents thereof. 

1. An aluminum casting alloy with anti tear characteristics comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about 3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc, about 0.18 to about 0.28 Zr, and the balance Al and impurities, substantially excluding Fe, Mn, and Si, said alloy characterized by a dispersion of L1₂ particles inoculating fcc grains, and η′-phase precipitates.
 2. The alloy of claim 1 wherein the mean grain diameter of the fcc grains is about 40 to 60 μm.
 3. The alloy of claim 1 wherein the η′-phase precipitates have a mean radius of less than about 5 nm.
 4. The alloy of claim 1 having less than about 0.0075 weight percent Fe, less than about 0.2 weight percent Mn and less than about 0.03 weight percent Si.
 5. The alloy of claim 1 having the following constituents in weight percent: about 5.8-6.8 Zn, 2.9-3.5 Mg, 1.0-1.2 Cu, 0.52 Sc, and 0.20 Zr.
 6. The alloy of claim 1 having the following constituents in weight percent: about 4.8-5.8 Zn, 2.7-3.3 Mg, 1.0-1.2 Cu, 0.55 Sc, and 0.25 Zr.
 7. The alloy of claim 1 having the following constituents in weight percent: about 4.0-5.0 Zn, 2.0-2.6 Mg, 0.52-0.72 Cu, 0.42 Sc and 0.25 Zr.
 8. An aluminum casting alloy with anti tear characteristics comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about 3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc, about 0.18 to about 0.28 Zr, and the balance Al and impurities, substantially excluding Fe, Mn, and Si, said alloy characterized by a freezing range of less than about 150° C., solidus temperature above about 490° C., and eutectic phase fraction above about 5% at the late stages of solidification.
 9. An aluminum casting alloy with anti tear characteristics comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about 3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc, about 0.18 to about 0.28 Zr, and the balance Al and impurities, substantially excluding Fe, Mn, and Si, said alloy characterized by a freezing range of less than about 150° C., solidus temperature above about 490° C., and eutectic phase fraction above about 5% at the late stages of solidification and a dispersion of L1₂ particles inoculating fcc grains, and η′-phase precipitates.
 10. An aluminum casting alloy with anti tear characteristics comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about 3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc, about 0.18 to about 0.28 Zr, and the balance Al and impurities, substantially excluding Fe, Mn, and Si, said alloy characterized by a freezing range of less than about 150° C., solidus temperature above about 490° C., and eutectic phase fraction above about 5% at the late stages of solidification, a dispersion of L1₂ particles inoculating fcc grains with a grain diameter of about 40 to about 60 μm, and η′-phase precipitates, and ambient yield strength from about 410 MPa to about 540 MPa. 